1. Field of the Invention
This invention relates in general to print output apparatus and methods, and more particularly to a method, apparatus and article of manufacture for modifying printing based upon direct on-the-fly media characteristic parameters.
2. Description of Related Art
In electronic print devices for printing, copying and desktop publishing systems, images are usually offered in electronic form, and are then referred to as electronic images. These electronic images can be stored on magnetic disk or transported via direct links or networks to the print devices. The creation of a page results in an electronic data stream or electronic file describing the several elements of the page layout in electronic format. This electronic page layout is usually expressed in a page description language. The electronic page layout comprises the data for each electronic image that must appear on the printed reproduction.
In order to render images such as pictures, letters, and characters from input information, halftone screens are used to cause the illusion of continuous-tone pictures when the display or print device is capable of producing fewer levels than are specified by the image. For example, input images are divided into a number of very small cells (called picture elements herein below) arranged in matrix form and each of the printed images is obtained as a whole by using picture element reproduction elements, where each picture element reproduction element corresponds to at least one or more picture elements. In general, according to this method, in most cases, logic circuits such as digital integrated circuits are used in electric circuits driving picture element reproduction elements.
In such a printing device, each electronic gray level, which may be an eight-bit number, for example, having a value N between 0 and 255, is converted to a visual density on the hard copy. This density, D, is defined as the negative base ten logarithm of the portion of reflected or transmitted light from incident light on the hard copy. The relation between the gray level N and the density D is strongly dependent on the output device. In order to get the same reproduction from the same electronic page layout on different output devices, the gray level N is not fed directly to the printing device, but rather a stimulus value S is applied to the printing device. The stimulus value S is derived from the gray level N in such a manner that after transformation of the stimulus S to a density D by the printing device, a specific relation exists between the gray level N and the resulting density D. This is called linearization of the printing device, and is thus achieved by establishing a device specific relation between the gray level N and the device stimulus S. This relation is referred to as a tone curve or a transfer function.
It is known in the art that most printing devices have a device specific transfer function incorporated, which is obtained by calibration. Each time when electronic images are printed on this device, the same device specific transfer function is applied to the gray levels of the electronic image.
Problems arise when the output mode of the output device changes without a corresponding change in print parameters. One output device can for example render electronic images on an output medium. The term medium or media indicates the physical material on which the output appears. Printers require reasonably high quality media in order to achieve high quality printing. As applied to paper, “print quality” is an assessment of at least one particular paper characteristic parameter. There are many influential objective factors bearing upon the print quality of print media.
There have been suggestions to consider characteristics of print media to optimize printing. For example, U.S. Pat. No. 4,835,573 issued May 30, 1989 to Charles Rohrer et al., which is assigned to International Business Machines Corporation, the assignee of the present invention, discloses using processing elements, such as microcode, to count the number of cut sheets fed from a supply bin and also to monitor the amount of bin travel. The data obtained is then used to estimate sheet thickness and weights to adjust machine operating elements which are sensitive to supply sheet quality parameters. However, actual media characteristic parameters are not directly measured on-the-fly.
U.S. Pat. No. 6,088,547 issued Jul. 11, 2000 to Michael Martin et al., which is assigned to Hewlett-Packard Company, suggests measuring a sympathetic response induced by the vibrating print media as it advances through the paper path. Sensors measure the vibration and associate the magnitude of vibration with the roughness of the paper, i.e., rougher paper vibrates more. The measured sympathetic response is then used to select a fuser temperature. However, actual media characteristic parameters are not directly measured on-the-fly. Rather, the roughness of the paper is obtained indirectly from vibration characteristics.
Co-owned U.S. patent application Ser. No. 09/876,507, filed Jun. 6, 2001, entitled METHOD, APPARATUS AND ARTICLE OF MANUFACTURE USING MEDIA ROUGHNESS AS A PRINT PARAMETER, the disclosure of which is hereby incorporated by reference, uses a fresnel lens and reflected light to directly measure the paper roughness. The directly measured paper roughness is then used to select the halftone screen.
Still, the use of other media characterization in setting printing parameters would allow high quality output to be obtained with a larger range of media than can be achieved with static printer parameters. The improvement of printing quality on media outside the “preferred” media types for the printer would provide more consistent and higher-quality overall printing. Additionally, support for a range of media wider than that currently supported would improve customer satisfaction by allowing customers more flexibility and cost savings on media used.
It can be seen then that there is a need for a method, apparatus and article of manufacture for modifying printing based upon direct on-the-fly media characteristic parameters. On-the-fly measurement and modification of parameters is important to print quality because print characteristics can change during the printing of a print job. For example, a parameter such as paper composition or texture can be non-constant through a roll of paper on which the print is marked. Additionally, on-the-fly adjustment reduces required operator interaction for high print quality.